Magnetic resonance imaging (MRI) is an imaging modality that is typically used to acquire images of internal features in a region of interest (ROI) of a person's body, and many and different MRI techniques have been developed to tailor MRI imaging to imaging different internal structures of the body. Modern MRI scanners used to acquire the MRI images may typically be configured to provide MRI images of internal features in an ROI of the body having spatial resolution as fine as 1 mm3 (cubic millimeter), and MRI scanners with microscopic resolution are available that resolve structural details of animal tissue as small as 0.1 mm3.
To acquire an MRI image of an ROI of a person's body, the person is placed in a relatively strong, uniform “polarizing magnetic field” to align spins of atoms in the person's body having magnetic moments along a same axis, conventionally referred to as a z-axis. A pulse of radio frequency (RF) energy is transmitted to illuminate the ROI and tilt the magnetic moments and thereby the spins of atoms in a thin slice of the ROI away from the z-axis by a predetermined tilt angle, generally equal to about 90°. Following “tilting”, conventionally referred to as “flipping”, by the RF pulse, the ROI is exposed to a perturbing magnetic field, which is also directed along the z-axis but is configured to have a desired time dependent spatial gradient along orthogonal x and y axes that are perpendicular to the z-axis. The superposition of the perturbing magnetic gradient field and the polarizing magnetic field creates a time dependent “composite” magnetic field in the slice that is a function of spatial coordinates along the x and y axes. The flipped atoms precess around the z-axis with Larmor frequencies that are proportional to the magnitude of the composite magnetic field at the x and y coordinates in the slice at which they are located and radiate RF signals, hereinafter also referred to as MRI signals, at the Larmor frequencies at which they precess. The MRI signals from the atoms at a given x-y location in the slice have intensities that are proportional to the concentration of the radiating atoms at the location and are received by an RF receiving antenna.
The received RF signals provide a discrete Fourier transform of the spatial concentration of the radiating atoms in the slice which is inverse Fourier transformed to provide a spatial concentration of the flipped atoms as a function of their spatial coordinates, x, y, and z. The spatial concentration provides an image of cross sections of organs and features of the body that are transected by the slice. A sequence of MRI images of adjacent, parallel slices in the ROI along the z-axis is acquired and processed to produce three dimensional (3D) images of organs and features in the ROI. Generally, resolution and signal to noise ratio (SNR) of the images improve with magnitude and uniformity of the polarizing magnetic field. High resolution MRI scanners may generate polarizing magnetic fields having magnitudes from about 3 Tesla to about 5 Tesla
To provide the strong polarizing magnetic field and the gradient magnetic field, a standard MRI scanner typically comprises a large tube or donut shaped housing that houses superconducting coils, which are excited to produce the polarizing magnetic field. Various additional current carrying coils that are excited to generate the gradient magnetic field are also housed in the housing.
The housing has a bore sufficiently large for receiving a human body, and during acquisition of MRI images of an ROI in a person's body completely surrounds the ROI. Typical standard MRI scanners therefore do not generally enable convenient access by a medical practitioner to perform a medical procedure, such as a surgery, at a target site of a patient's body while the target site is being imaged by the MRI scanner. As a result, whereas standard MRI scanners are often, and often indispensably, used to acquire high resolution images of a target site of a person's body in preparation for a medical procedure, the scanners are generally not readily used to acquire real time MRI images of the region during the procedure to track and update changes caused by the procedure. Often when used during a medical procedure, a patient is intermittently inserted and removed from the bore of a conventional MRI scanner to track progress of the procedure.
MRI scanners and MRI procedures referred to as intraoperative MRI (iMRI) scanners and interoperative iMRI procedures have been developed to improve usability of MRI imaging during medical procedures. The iMRI scanners and iMRI procedures may provide substantially real time MRI images of a target site during a medical procedure performed at the target site. Typically, an iMRI scanner generates polarizing magnetic fields having relatively small magnitudes, which may for example be as low as 0.15 to 0.5 Tesla, To enable advantageous physical access to a target site, the iMRI scanner may comprise a split housing comprising two donut shaped component housings that have relatively short bores and are placed facing each other on opposite sides of an access space. The component housings comprise superconducting coils that are excitable to generate a polarizing magnetic field in the access space. A patient may be placed in the iMRI scanner with a target site of the patient's body located in the access space so that a medical practitioner may have at least partial physical access to the target site to perform a medical procedure while the site is being imaged by the iMRI scanner to acquire mages for tracking and monitoring progress of the medical procedure.
Whereas an iMRI scanner may enable at least limited access to a target site of a person's body to perform an operation at the target site while the iMRI scanner images the target site, iMRI scanners, such as those developed for intraoperative MRI brain surgery often operate with low polarizing magnetic fields that are on the order of 0.15 Tesla and have a smaller field of view (FOV) than standard diagnostic MRI scanners. As a result, iMRI images that they provide generally have lower spatial resolution than standard diagnostic MRI images.
For example, prior to open brain surgery, relatively large FOV, high spatial resolution anatomical MRI, functional MRI (fMRI) and diffusion tensor MRI (DTI) images of the brain may be acquired using a standard high field MRI scanner to reference and direct navigation in the brain during surgery. The high resolution images may image the whole brain and skull and resolve features of the brain and skull having characteristic dimensions less than or equal to about a millimeter. After craniotomy to perform the surgery, it is generally impractical to acquire high resolution fMRI or DTI images, and an iMRI scanner may be used to acquire iMRI images of the brain during the surgery to aid in performing the surgery. However, because of a relatively small FOV of the iMRI scanner, the iMRI images may image only a part of the brain, and may have a slice thickness in a range from about 3 mm to about 5 mm, which is substantially thicker than that of the preoperative fMRI and DTI images. In addition, features of the brain imaged in the iMRI images may be deformed as a result of disturbance of the brain tissue by the surgery, making it difficult to accurately identify and locate the features during surgery. As a result, use of the iMRI images may be limited.